When it is detected that the temperature of a fuel cell stack is below water freezing temperature (below freezing point) at the start of a fuel cell system, a control map for the normal start is changed to a control map for the start at temperatures below freezing point, and the fuel cell system is started in accordance with the control map for the start-at temperatures below freezing point. Then, if it is detected that the temperature of the fuel cell stack is above the freezing temperature, the control map for the start-at temperatures below freezing point would be changed to the control map for the normal start, and the fuel cell system would be started in accordance with the control map for the normal start. In this case, a start method may be changed based on the remaining amount of product water in the fuel cell (see, e.g., Patent Document 1).
When starting the fuel cell system at temperatures below freezing point in this way, there is also technique for improving the start performance by performing a rapid warm-up operation for rapidly increasing the temperature of the power generating section to exceed temperatures below freezing point. Such method includes, for example, a method of controlling the cooling water pressure at the inlet of a fuel cell stack lower than the pressure used for operating the fuel cell system in a steady state before completing the fuel cell stack warming-up, and controlling the cooling water pressure at the inlet of the fuel cell stack to the pressure used for the operation in a steady state after completing the fuel stack warming-up.
Furthermore, as a technique of preventing product water from freezing inside a fuel cell when starting the fuel cell in cold climate areas, there is a technique which allows a temperature sensor to detect the internal temperature of the fuel cell. In this case, cooling water pump is controlled to be in a stop state when the internal temperature of the fuel cell is 0° C. or less, and when the temperature exceeds 0° C., the driving amount thereof increases as the internal temperature rises (see, e.g., Patent Document 1).    Patent Document 1: Japanese laid-open patent publication 2008-147139 A    Patent Document 2: Japanese laid-open patent publication 2003-36874 A